Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 375,168	$ 113,938
Investments, available-for-sale	291,596	369,616
Accounts receivable	4,510	663
Unbilled accounts receivable	20,597	22,749
Inventory	2,665
Prepaid expenses and other current assets	10,147	9,820
Total current assets	704,683	516,786
Investments, available-for-sale	83,666	64,406
Property and equipment, net	37,822	38,361
Operating lease right-of-use assets, net	71,903	72,753
Restricted cash	5,168	5,166
Other assets	10,381	10,222
Total assets	913,623	707,694
Current liabilities:
Accounts payable	8,468	4,793
Accrued expenses	73,961	88,706
Current portion of operating lease liabilities	7,169	6,823
Current portion of deferred revenue	8,690	6,160
Total current liabilities	98,288	106,482
Operating lease liabilities, net of current portion	87,667	89,126
Deferred revenue, net of current portion	36,809	39,913
Other long-term liabilities	7,928	7,814
Total liabilities	230,692	243,335
Commitments (Note 15)
Stockholders' equity:
Preferred stock, $0.001 par value; 5,000,000 shares authorized; no shares issued and outstanding
Common stock, $0.001 par value; 120,000,000 shares authorized; 54,168,533 and 49,272,223 shares issued and outstanding at March 31, 2020 and December 31, 2019, respectively	54	49
Additional paid-in capital	1,739,140	1,412,083
Accumulated other comprehensive loss	(69)	(2,535)
Accumulated deficit	(1,056,194)	(945,238)
Total stockholders' equity	682,931	464,359
Total liabilities and stockholders' equity	$ 913,623	$ 707,694